When serializing msgpack, only consider codec tag

When serializing structs with msgpack, only consider type tags of
`codec`.

Hashicorp/go-msgpack (based on ugorji/go) defaults to interpretting
`codec` tag if it's available, but falls to using `json` if `codec`
isn't present.

This behavior is surprising in cases where we want to serialize json
differently from msgpack, e.g. serializing `ConsulExposeConfig`.

plugins/base/plugin.go

@@ -65,6 +65,10 @@ var MsgpackHandle = func() *codec.MsgpackHandle {
 	h.BasicHandle.TimeNotBuiltin = true
 
 	h.MapType = reflect.TypeOf(map[string]interface{}(nil))
+
+	// only review struct codec tags - ignore `json` flags
+	h.TypeInfos = codec.NewTypeInfos([]string{"codec"})
+
 	return h
 }()